The invention is related to the use of multi-layer sheet materials containing a polymeric base film inter alia as labels and label stocks, novel multi-layer sheet materials and a method for preparing such multi-layer sheet materials. The invention is also related to the use of a polymeric film in laser-cuttable and/or laser-markable articles.
Various markable materials in the form of self-adhesive sheets or films are known, which may be marked, for example, by means of a laser beam. Such markable materials are predominately used in the commercial sector as nameplates, rating plates, information plates, identification plates, inventory labels, barcode data carriers, animal tags and the like. Laser-markable materials have also found application in optically readable, e.g. authenticatable, articles, such as identity cards, teller cards and the like.
For commercial purposes it is desirable that the marked materials, e.g. laser-marked labels, are resistant to the effects of aging, chemicals, high temperatures and natural weathering. The specific properties required are linked to the final application and the demands of the user. For certain uses, for example identification plates of automobiles, it is desirable that the laser-markable materials, such as labels, are tamper resistant. Under the term tamper resistance is to be understood that in its final application the laser-markable material, e.g. label, cannot be removed without damage.
Laser-markable adhesive labels containing PVC are mostly used. Such labels typically contain a base film or layer with an adhesive layer on one surface of the base film and a top film or layer on the opposite surface of base film. The base and top films are pigmented in contrasting colors. To form an image, the top film is ablated and thus partially removed by means of the laser beam, exposing the base film of a different color. Upon this type of marking corrosive gases may be formed, inter alia due to the vaporization of the top film, which contains a halogen-containing material.
The German utility model G 81 30 861 discloses a multi-layer label consisting of two layers of lacquer. These layers of lacquer consist of e.g. polyurethane cross-linked by radiation. The preferred lacquer consists of a mixture of hexanediol bisacrylate and a commercially available polyurethane acrylate which is composed of a long-chain polyester diol, an aliphatic diisocyanate and terminal acrylic moieties.
Laser-markable labels of this kind are obtainable under the trade name TESA 6930 (Company Beiersdorf AG, Hamburg, Germany). By the process of radiation cross-linking the possibility of adjusting mechanical properties is limited. Consequently the labels show little flexibility and do not conform to irregular surfaces.
Also commercially available are laser-markable adhesive labels of the company W. H. Brady Company, Milwaukee, U.S.A. These labels are predominately made of biaxially orientated polyesters. These labels are not tamper-resistant. These adhesive labels consist of a top film and a base film, both of which have been dyed in different colors.
In Chemical Abstracts 111 (18) 153979c (JP-A-90 120042) there have been described heat-resistant self adhesive (pressure-sensitive) films which are used as laser-markable labels. 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid anhydride 3,3xe2x80x2-thiodianiline polyamino acid is mixed with carbon black dried and treated at 180xc2x0 C. for 60 minutes and then at 250xc2x0 C. for six hours. One surface of the resulting layer was coated with a solution containing titanium dioxide, and the layer was also treated at an elevated temperature. The reverse surface of the layer was provided with a silicone-based pressure-sensitive adhesive.
Nagafushi, in U.S. Pat. No. 4,772,512 discloses a composite film for bar code labels comprising first and second heat resisting non-water absorbing synthetic resin films. On the first film of the composite film, bar codes can be reproduced by means of a high speed drying process electrostatic copier. As the first and second heat resisting, non-water absorbing synthetic resin films in the composite film polyester films, polyimido films, polycarbonate films, cellulose ester films and polyamide films were used.
U.S. Pat. No. 5,688,573 discloses a halogen-free acrylic urethane sheet material comprising a polymer comprising components A, B and C wherein component A comprises a copolymer of (i) a monoester of acrylic or methacrylic acid and an aliphatic diol having 2 to 8 carbon atoms and (ii) a ester of acrylic or methacrylic acid and an aliphatic alcohol having 1 to 8 carbon atoms, and optionally a vinyl aromatic monomer as well as N-vinylpyrrolidone or N-vinylcaprolactame, said component A having a Tg of 30xc2x0 C. to 100xc2x0 C., component B comprising an aliphatic polyester diol with a weight-average molecular weight less than 20,000 and component C comprising a polyfunctional isocyanate or blocked polyfunctional isocyanate. The acrylic urethane sheet material is radiation, e.g. laser, markable.
CA 2,155,233 discloses an authenticatable laminated article comprising an inner, laser beam markable core layer, two protective layers, an optical interference layer and an adhesive layer, The core layer may be any material known in art which can be discolored by a laser beam.
EP 0 732 678 and EP 0 688 678 disclose respectively a multi-layer or single-layer self adhesive label containing a base layer, i.e. film, made of plastic, which contains an additive which exhibits a color change on irradiation with a laser. Preferred plastics for the base layer are e.g. polyester, poly(meth)acrylate, polycarbonate and polyolefins, unsaturated polyester, epoxy-, polyester- and urethanacrylate, in particular those of G 81 30 816.
A laser-markable label stock in which the laser marking or imaging is based on a color change is commercially available under Color Laser Film black/white (Company Schreiner Etiketten und Selbstklebetechnik Gmbh and Co, Oberschliexcex2heim bei Mxc3xcnchen, Germany). Said label stock contains a single polyester base film with an adhesive layer on one surface of the film.
In the production of such laser-markable labels, the label form is typically cut from a sheet material, a film or a label stock using the laser beam, which also serves to create the image on the label. This provides greater flexibility in the production process and avoids the costly establishing of die-cutting equipment next to the laser beam equipment. However one of the limiting factors in the rate of such manufacture of labels is the speed at which the label form can be cut from the sheet material or film or label stock. Also in the production of laser-marked articles, where an image is produced by laser cutting, e.g. etching, engraving or ablation, the rate at which the corresponding film or layer can be cut by the laser can be a time limiting factor for the laser marking.
Thus, it would be advantageous to develop and/or apply sheet materials comprising a polymeric backing film which show rapid laser cuttability and at the same time exhibit properties expected for use of such materials in the commercial sector.
In particular, it is desirable to provide sheet materials for various purposes, e.g. for use as laser-markable adhesive labels or label stocks, as well as polymeric films for use in laser-markable and/or laser-cuttable articles, which show rapid laser cutting times and/or excellent mechanical properties and resistance to severe environmental stress like heat, solvents and weathering and do not generate halogen-containing corrosive gases upon laser cutting.
It has been surprisingly found that polymeric film products made of aminoplast cross-linked hydroxy-functional resin exhibit advantageous properties, and in particular, excellent laser cutting performance.
Accordingly, one aspect of the present invention is the use of a halogen-free multi-layer sheet material comprising
(i) a base film of a polymer comprising the reaction product of a precursor mixture comprising at least one hydroxy-functional resin and an aminoplast cross-linking agent;
(ii) an adhesive layer; and
(iii) optionally, a release liner, as a label or label stock.
A second aspect of the present invention is the use of a halogen-free multi-layer sheet material as described above in the manufacture of a laser cuttable and/or laser markable article.
A third aspect of the present invention is the use of a halogen-free film of a polymer comprising the reaction product of a precursor mixture comprising at least one hydroxy-functional resin and an aminoplast cross-linking agent in a laser-cuttable and/or laser-markable article.
A further aspect of the present invention is the provision of a halogen-free multi-layer sheet material comprising
(i) a base film of a polymer comprising the reaction product of a precursor mixture comprising a first hydroxy-functional resin, a second hydroxy-functional resin and an aminoplast cross-linking agent;
(ii) an adhesive layer; and
(iii) optionally, a release liner,
wherein the first hydroxy-functional resin is a hydroxy-functional acrylic resin and the second hydroxy-functional resin is selected from the group consisting of hydroxy-functional polyester resins, hydroxy-functional polyether resins, hydroxy-functional epoxy resins, cellulose derivatives and phenol formaldehyde resins.
Coating systems comprising aminoplast cross-linked hydroxy-functional resins are known from U.S. Pat. Nos. 5,977,256, 5,593,785, EP 0 909 288, U.S. Pat. Nos. 3,843,390 and 5,780,559. Said prior art documents relate, however, exclusively to coating technology and are silent with respect to self-adhesive sheets or films as well as laser-cuttable and/or markable materials, sheets, films, labels or label stocks.
EP 0 359 532 discloses a protective overlay film comprising a protective layer made from a mixture of at least one hydroxy-functional acrylic polyol and at least one polyfunctional curing agent and an adhesive layer on inner surface of the protective layer. This document however is silent with respect to labels or label stocks as well as laser-cuttable and/or markable materials, sheets, films, labels or label stocks.
The multi-layer sheet materials according to the invention as well as the use of the polymeric films and multi-layer sheet materials according to the invention exhibit advantageous mechanical properties and/or allow a more rapid laser cutting and thus manufacture of articles, such as labels or precut-label stocks, which are form-cut by a laser. Also for articles, which are marked by laser etching or ablation, the production of the marking or image may require less time.
Advantageously, the properties of the polymeric film can be adjusted within definite ranges to meet specific user-demands and field of applications, especially in regard to excellent mechanical properties and resistance to severe environmental stress like heat, solvents and weathering as required for commercial use. Also certain preferred sheet materials or films of the invention provide sheet materials or films which exhibit properties suitable for tamper resistant labels or the like.
The multi-layer sheet materials and films in accordance with the invention can be laser cut or marked without the formation of halogen-containing noxious gases and thus exhibit in this regard environmental compatibility.
The present invention also provides a method of preparing a multi-layer sheet material, the method comprising the following steps:
a) providing a precursor mixture comprising a first hydroxy-functional resin, a second hydroxy-functional resin and an aminoplast cross-linking agent, wherein the first hydroxy-functional resin is a hydroxy-functional acrylic resin and the second hydroxy-functional resin is selected from the group consisting of hydroxy-functional polyester resins, hydroxy-functional polyether resins, hydroxy-functional epoxy resins, cellulose derivatives and phenol formaldehyde resins;
b) providing a film of the mixture and curing the mixture to form a base film; and
c) applying an adhesive layer onto the base film and, optionally, covering the adhesive layer with a release liner to form the sheet material.